RESISTANT MECHANISMS OF CISPLATIN IN HUMAN LUNG ADENOCARCINOMA CELL LINE As49DDP

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1 Chinese Journal of Cancer Research 9(3): Z RESISTANT MECHANISMS OF CISPLATIN IN HUMAN LUNG ADENOCARCINOMA CELL LINE As49DDP -," 1 Zhan Maocheng ~).~[e~ Xu Guangwei r ~'~6 Liu Xuyi ~11;~.s Cai Peng ~.~ School of Oncology, Beijing Medical Universi~'. Beijing To study the resistant mechanisms of cisplatin in human lung adenocarcinoma cell line As49DDP. A~49DDP cells was established by stepwise increasing concentration of cisplatin (CDDP) in medium. Interstrand cross- linked DNA (ICL) was measured by ethidium bromide fluorescence assay. The intracellular and intranuclear accumulation of cisplatin was measured by atomic absorption spectrometry. The removal of GS-X was determined by FCM and fluorescence microscopy. Results: The As49DDP cell line was 8.9-fold resistance relative to the parental As49 cell line. The formation of ICL in A~9 was 6.28 times higher than that in A~gDDP cells. The intracellular and intranuclear accumulation of cisplatin in A~ cells was 5.9 times and 4.1 times higher than that in As4,DDP cells, respectively. The ability of GS-X pump pumped GS-X complex (GS-Pt) in As49DDP cells was higher than that in As49. The repair rate in A~J)DP cells was 2 times higher than that in A~. Conclusions: Decreased accumulation and increased export of cisplatin might be the main mechanism of cisplatin resistant As49DDP cells while the enhanced repair capacity of DNA may play a role in CDDP resistance. Key words: Resistance mechanism, Human As4,, Cisplatin, lnterstrand cross-link, Accumulation, DNA repair capacity. Cisplatin (CDDP) is one of the most effective antitumor agents and has been used broadly in the Accepted June 28, 1997 treatment of lung, testicular, ovarian, head and neck cancers) However, its usefulness is limited by the rapid development of acquired resistance, and which is often regarded as one the causes of chemotherapeutic failure. Therefor, an understanding of the mechanisms underlying the process of acquired (CDDP) resistance is the main problem to be solved preclinically. The resistant mechanisms of cisplatin are considered to be as following: (a) decreased accumulation of cisplatin; (b) enhanced repair capacity of cisplatin-dna adducts; (c) increased inactivation of cisplatin by detoxification system, etc. 2'3 In an attempt to understand the resistant mechanism of cisplatin further, a cisplatin-resistant human lung adenocarcinoma cell line Asa9DDP cells was established by stepwise increasing concentration of CDDP in cell culture medium. The formation of interstrand cross-link, the intracellular and intranuclear accumulation of cisplatin, the excretion of GS-X, the repair capacity in As49DDP cell lines as well as in its parental cell line A549 was studied. Materials MATERIALS AND METHODS RPMI-1640 was purchased from Gibco; Fetal bovine serum was obtained from Institute of Hematology, Tianjin; Cisplatin was purchased from Shandong Qilu Pharmaceutical Factory; Thiazollblue, ethidium bromide, sarkosyl, nonidet P-40 were purchased from Sigma; Monochlorobimane (MCB) 178

2 was purchased from Calbiochem. Cell Culture A549 cell line was incubated in RPMI-1640 medium containing 15% fetal bovine serum and with a stepwise increasing concentration of CDDP for 12 months. The selected A549DDPcell line was incubated in the medium containing 12 I.tMol.L ~ CDDP for more than 4 months, and the resistant phenotype of the A549DDP cell was stable for more than 5 months in the absence of CDDP. Cytotoxicity The cytotoxicity of CDDP on A549 and As~gDDP was determined by MTI" assay. The ratio of IC50 of the resistant cell line A549DDP/that of the sensiiive cell line A549 was designated as resistant fold. Determination of lnterstrand Cross-link (ICL) Renaturation of DNA after denaturation and rapid cooling is related to the formation of DNA cross-links. Double stranded renatured DNA were stained selectively by ethidium bromide and measured by fluorescence assay (EFA). 4 The A549 and A549DDP cell lines were exposed to various concentrations of CDDP for 1 h., washed with hank's balanced solution, after lysed, ICL was determined by fluorescence assay. For repair study, the cells were incubated with 200 ~MoI.L -] CDDP for 2 hr. After removed the drug, the cells were maintained in fresh medium free of CDDP, then the ICL was measured at 0 and 24 hr., the ratio between the ICL at 24 hr. and at 0 h. was designated as repair rate. Retention: After the A549 and A549DDP cells were treated with 60 t.tmol.l -] CDDP for 1 h., the cells were washed with cold PBS three times at 4 ~ and then maintained in fresh medium without drugs, the concentrations of cisplatin were measured at different intervals. We take the concentration at 0 h. as 100%, the relative content at other time was compared to it. Removal of GS-X Monochlorobimane (MCB), a nonfluorescence compound, is specially conjugated with glatathione (GSH) in the cell by the action of glutathione S- transferase, and the resulting glatathione-s conjugate (GS-X) exhibits intense fluorescence. GS-X was excreted by GS-X export pump. 6 GSH reacts with CDDP, forms GSH-cisplatin complex (GS-Pt) which was also excreted by the GS-X pump. 2 Therefor, we use MCB as a indicator to study the excretion of GS- Pt indirectly by FCM and fluorescence microscopy. 7 l cells/ml were incubated with 20 lam MCB at 37 ~ for 20 min., washed and incubated in the MCB- free medium for 0, 45, 90 rain. The fluorescence intensity of the cells was analysed by FCM and observed under fluorescence microscopy. Cytotoxicity Results RESULTS After have exposed to CDDP. The ICso of both cell lines was calculated. The ICs0 for A549 and A549DDP was 9, 80 lamol.l -I, respectively. The resistant fold was 8.9 (Figure 1). Accumulation of Intracellular Concentration of Cisplatin and lntranuclear I00 8O L j, Accumulation: 2 A549 and A549DDP cells were treated with various concentrations of CDDP for 1 h., washed twice and solubilized in 0.5 ml nitric acid at an electric dry plate (260 "C). The final volume was adjusted to 200 I11 and measured by atomic absorption spectrometry: For determining the accumulation of CDDP in intranuclear, the treated cells were washed twice, the cells pellets were resuspended in NTE buffer (0.1 M NaC1, 0.01 M Tris-EDTA) and lysed with 5% Np40. Nuclei were obtained by centrifugation i i i I Cisplalin / p.moi.l 'J Fig. 1. Relationship between cisplatin concentration and survival rate of A549 and A549DDP cell lines, n=3. ~ s 179

3 Formation of ICL The formation of ICL in parental A549 cells was 6.28-fold higher than that of the resistant A549DDP cells. The ICL was directly proportion to the concentration of CDDP (Figure 2). Accumulation of Cisplatin Intracellular and intranuclear concentrations of cisplatin in As49DDP cells was 5.9-fold and 4.1-fold lesser than that in A549 cells, respectively (by the ratio of slope of regression line). Both of them were also directly proportional to the concentrations of CDDP (Figure 3). The CDDP content exported from A549DDP cells more quickly than that from A549 cells, especially during the first 4 h. There is only a little difference between the A549DDP cells and A549 cells about the excretion of cisplatin from nucleus (Table 1 ).,The nuclei-cytoplasm ratio (N/S) in A549DDP cells was 50% that in A549 (Table 2). Removal of GS-X After incubated the A549 cells and the A549DDP cells with MCB, the florescence intensity of the A549DDP cells decreased dramatically during 0-90 rain., although the initial fluorescence intensity in resistant A~,,9DDP cells was even higher than that in A~9 cells (Figure 4). This was also showed by FCM as a shift of the fluorescence intensity peak (Figure 5). -T 701 / 9-0- A~ DDP 50 3O 10 -O- A~ Cisplatin/imloI.L "j Fig. 2. Relationship between cisplatin concentration and intrastrand cross-link rate in Ara9 and A~gDDP cell lines. n=3,.x"a: s The Repair of ICL The repair rate in the A549DDP cells is 2 times higher than that in A549 cells (Table 3). Table I. Relationship between the relative cellular ( C) and nuclear (N) retention of cisplatin and the time ofpost-cisplatin in A_~9 and A549DDP n=3,.r + s Oh 4 h 8 h 12 h A~,9(C) 519.8:1:78 (100%) (85.7%) (76.2%) (64.7%) A~gDDP(C) (100)% ~ 13 (56.8%) (43.31%) 78.7:i:9.8 (22.2%) A549 (N) (100%) (87.7%) 84:t:9.1 (85.3%) 72.3:1:5.3 (73.8%) A549DDP (N) (100%) (85.3%) J:2.89 (79.1%) 10.3:t:0.74 (61.2%) Table 2. lntracellular ( C) and intranuclear (N) accumula- tion of cisplatin after exposed of the A~4~ and As49DDP cell lines to 60 limol.l t cisplatin for I h, n=3, x s, ng 2 cells 1 A549 A~49DDP C " N 98.5_ ' N/C *P<0.01 DISCUSSION The mechanism of resistance to cisplatin is still controversial, although several kinds of processes have been proposed. For elucidation of the mechanism of CDDP resistance, we examined the formation of DNA interstrand cross-links (ICL)0 and its repair capacity in CDDP-resistant human non-small cell lung cancer cells (AsagDDP/8.9-fold resistance). 180

4 We measured the intracellular and intranuclear concentrations of CDDP and the N/C ratio. We also examined the pumping of GS-X in both of the As49DDP and the A549 cell lilies. Our results showed a 6.28-fold reduction of formation of ICL in the resistant As49DDP cells than that in the sensitive A549 cells. A 6.28-fold reduction is close to the 8.9-fold resistance of A s49ddp to A549, therefore it suggests that ICL was the main cytotoxical form of CDDP. These results are very similar to those of Bungo's work. s Table 3. Percentage of lcl in the A_~9 and the A 3~gDDP cell lines after exposed to cisplatin 100 p.mol.e / for 2 h, n= 3, xat s. 0 h 21 h Repair (%) A As49DDP 0.468_ _ ' 9 P<0.01 A,j~DDP A~, O- A~ (C) (I zooo 45 min.s Cisplatin j.~moi.l r 90 rain Fig. 3. Relationship between intracellular (C) and intranuclear (N) accumulation of cisplatin and cisplatin contation in As49 and A~gDDP cell lines, n=3,.~_ s Fig. 4. Fluorescence microscopic analysis of the A5~9 and A~gDDP incubated with monochloribimane. AugDDP 0 min 1 L li It I J\!! If 45 min! I I I000 MCB MCB Fig. 5. Flow cytometric analysis of the fluorescence of A549 and A s49ddp incubated with monochloribimane. In an attempt to elucidate the cause of reduction of ICL, a 5.9-fold decreased intracellular accumulation of cisplatin was found in the resistant As49DDP cells than that of the A549 cells, which suggested that the decreased accumulation of CDDP caused the reduction of ICL. 181

5 The decreased accumulation of cisplatin may be the results of enhanced export and decreased uptake. Our results shew the former was the main mechanism. Apart from decreased accumulation of cisplatin, the lower nuclei-cytopalsm ratio suggested that the CDDP was obstructed from entering nuclei in the resistant As49DDP cells. The 2-fold increased repair activity in the resistant A549DDP cells also suggested it might make some contribution to CDDP resistance. We have showed that the A549DDP resistance related to GSH levels, GST contents in our previous study. 9 In this investigation, our results suggested that decreased accumulation, enhanced export of DNA-Pt consisted one of the main resistant mechanisms while the increased repair activity also play some roles in it. REFERENCES 1. l.,oehrer PJ, Einhom L. Cisplatin. Ann Intern Med 1984; 100: Gilbert C. Cellular response to cisplatin. J Biol Chem 1994; 269: Johnson SW, Shen DW, Pastan I, et al. Crossresistance, cisplatin accumulation, and cisplatin-dna adduct formation and remova/ in cisplatin-sensitive and -resistant human hepatoma cell lines. Exp Cell Res 1996; 226: Jong S, Zulstra JG, Bosscha HT, Mulder NH, et al. Detection of DNA cross-links in tumor cells with the ethidium bromide fluorescence assay. Int J Cancer 1986; 37: Levy E, Baroche C, Barret JM, et al. Activated ras oncogene and specially acquired resistance to cisplatin in human mammary epithelial cells: induction of DNA cross-links and their repair. Carcinogenesis 1994; 15: Oude ER, Bakker CKK, Roelofsen H, et al. Hepatology 1993; 17: Ishikawa T, Wright CD, lshizuka H. GS-X pump is functionally overexpressed in cis-diamminedichloro- platinum ()-resistant human leukemia HL-60 cells and down-regulated by cell differentiation. J Biol Chem 1994; 269: Bungo M, Ujiwara Y, Kasahara K, et al. Decreased accumulation as a mechanism of resistance to cis- diamminedichloroplatinum () in human non-small cell lung cancer cell lines. Relation to DNA damage and repair. Cancer Res 1990; 50: ~lll~, ~tj:~..~{~,, ~t~, ~. ~,~.~.K~l,t~-~tl~ ~ I~J ~ ~f./,). ~ll~j.j ]~. J~.. ~ ~ ~I~ ~ ~Ii ~ 1995; 22: